There are people who buy dollar coins straight from the US mint (with free shipping) with credit cards, deposit the coins to pay off the bill, and accumulate rebates and frequent flier miles. IIRC one guy bought an appreciable fraction of a million dollars worth of coins, a few grand at a time, and got a similarly ridiculous number of frequent flier miles out of it.

One reason this is considered "real life hacking": it's an exploit of a bug. Credit card companies generally exclude "buying cash" from their rewards programs ("some restrictions may apply"), but some of them failed to account for dollar coins as a form of cash.